This invention relates to a vertical deflection amplifier.
It has been known to modulate the horizontal scanning waveform with a vertical scanning waveform to correct for keystone distortion generated, for example, in the operation of an iconoscope. The vertical rate signal introduces a tilt into the vertical lines of the scanned image at the vertical rate, thus correcting for skew in the vertical direction.
It would be desirable to modulate the vertical scanning waveform with the horizontal scanning waveform in order to introduce a tilt into the horizontal lines of the scanned image and thus correct for horizontal skew. If both horizontal and vertical skew corrections are applied simultaneously, the resulting image can be rotated in the image plane. The ability to rotate the image is useful, for example, in a color television camera where it is necessary to correct for the misregistration between the images produced by the three image pickup tubes.
The introduction of vertical frequency components into the horizontal deflection amplifier can be accomplished without difficulty because the relatively low frequency vertical components are readily accommodated by an amplifier designed to operate at the horizontal frequency. The vertical rate retrace voltage pulse generated by the application of the vertical scanning waveform to the horizontal deflection coil is relatively small and thus does not tax the dynamic range of the horizontal deflection amplifier.
However, the introduction of horizontal scanning components into the vertical deflection waveform poses serious problems for the vertical deflection amplifier. Vertical deflection amplifiers need only reproduce a 60 hertz sawtooth waveform and are therefore typically of limited bandwidth. The horizontal deflection frequency, being the 263rd harmonic of the vertical deflection frequency, will pose some frequency response difficulties for the vertical deflection amplifier. The most serious frequency response problem is created by the horizontal retrace pulse component which has a fundamental frequency of 80 kilohertz. In addition, in order to produce the required fast rising edges and squared-off corners of this retrace pulse, a bandwidth of approximately 0.5 megahertz is required in the amplifier.
The horizontal retrace pulse will also severely tax the dynamic range requirements of the vertical deflection amplifier. At the low frequencies utilized in vertical deflection, the vertical deflection coil will appear primarily resistive. At 80 kilohertz, this coil will appear highly inductive and thus exhibit a high impedance. It is necessary that a sawtooth current be driven through the vertical deflection coil at the horizontal rate in order to accomplish the horizontal skew correction or image rotation. This requires that the amplifier have the necessary dynamic range to generate the resulting retrace voltage pulse.
It is possible to solve this problem by utilizing a second horizontal deflection amplifier to drive the vertical deflection coil. Horizontal deflection amplifiers for television cameras are typically class A amplifiers. Class A amplifiers are of low efficiency which wastes energy, requires high capacity power supplies, and places a higher heat burden upon the equipment.
The use of a class B vertical drive amplifier would solve the efficiency problem posed by the class A amplifier. A suitable class B amplifier having emitter follower output stages would require a high voltage gain driver amplifier stage. Gain bandwidth limitations of the driver stage make such designs impractical.